1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and more particularly to a liquid crystal display apparatus in which two storage capacitor lines are provided for each pixel row.
2. Description of Related Art
Liquid crystal display apparatuses, which can be made thin and small and which can be configured to consume very little power, are now found in many electronic devices as displays. Such liquid crystal displays (hereinafter referred to as “LCDs”) have a structure in which two substrates having electrodes formed on the respective surface sides facing each other are attached together with liquid crystal sealed between the two electrodes. The LCDs perform display by applying a voltage signal between these electrodes to control alignment of the liquid crystal whose optical characteristics change depending on the alignment state and thereby control transmissivity of light from a light source.
In known devices, a problem of image persistence, in which the alignment state of liquid crystal molecules is fixed when a direct current voltage is continuously applied between the electrodes formed on the sides of the substrates facing each other, is common. In order to deal with this problem, an alternating current voltage signal whose polarity with respect to the reference voltage is periodically inverted has been conventionally used as a voltage signal for driving the liquid crystal.
In a liquid crystal display apparatus in which a plurality of pixels are disposed in a matrix, with respect to the timing of inverting the polarity of a liquid crystal driving voltage signal, inversion for each frame, inversion for each one vertical scanning (1V) period (or one field period), inversion for each one horizontal scanning (1H) period, and inversion for each one pixel (one dot) period are known.
Active matrix type LCDs of higher display quality, particularly those displaying moving images with a higher display quality than other driving methods, commonly include a plurality of pixels arranged in a matrix within the display region. Each pixel includes a switching element such as a thin film transistor (TFT), a storage capacitor, and a liquid crystal capacitor formed by a pixel electrode and an opposing electrode which is opposed to the pixel electrode with a liquid crystal layer sandwiched between them. With such a structure, the polarity of a display voltage VP to be applied to the individual pixel electrode connected to the TFT and provided for each pixel is periodically inverted with respect to a voltage signal (a common voltage signal) Vcom to be applied to the side of the opposing electrode (common electrode). Conventionally, by periodically inverting the polarities of both the opposing electrode and the storage capacitor, deterioration of the liquid crystal is prevented and also the voltage amplitude of the H driver which outputs a data signal to each data line is decreased for achieving reduced power consumption of the H driver.
However, in the horizontal inversion counter-electrode AC driving in which the polarity of a video voltage signal applied to each data line is inverted for each one horizontal period, because the polarities of the opposing electrode and the voltage at all the storage capacitor lines are inverted for each one horizontal period, a capacitive load at the opposing electrode and all the storage capacitor lines and the resulting power consumption is still high.
Accordingly, in order to achieve further reduction in power consumption, the present applicant proposed, in Japanese Patent Laid-Open Publication No. 2000-81606, a driving method in which separate lines are provided for supplying a voltage to the opposing electrode and the storage capacitor, respectively, and the voltage (Vcom) of the opposing electrode having a large capacitive load is fixed whereas the polarity of the voltage at all the storage capacitor lines are inverted, whereby electric current and voltage of the H driver is decreased (hereinafter referred to as an “SC driving”.
Here, the inversion driving methods for periodically inverting the polarity of the voltage to be applied to liquid crystal are roughly divided into two types: line inversion driving and dot inversion driving. In line inversion driving for inverting the polarity for each line in the vertical or horizontal direction, the voltage amplitude of a video voltage is reduced to half that of the dot inversion driving by inverting Vcom together with the data signal. In this method, however, flickering caused by slight differences in brightness between the positive and negative electrode lines are noticeable, and, because positive or negative polarities are arranged in line in the horizontal or vertical direction, it is likely that a horizontal or vertical line appears as flicker when the inversion frequency (i.e. a frame rate when the polarity is inverted for each frame) is reduced. Thus, the line inversion driving requires a high frame rate. On the other hand, in dot inversion driving in which the opposite polarity is applied to all the adjacent pixels on the upper and lower and right and left sides, Vcom (opposing electrode) is fixed and a voltage whose polarity is inverted to either positive or negative with respect to the reference voltage Vcom is applied as a video voltage. Accordingly, for a display apparatus having integrated pixels, while flickering is not noticeable even at a low frame rate because the positive and negative polarities are evenly mixed, a large voltage amplitude is required. Consequently, as the power consumption of liquid crystal depends on the driving frequency and the voltage amplitude, reduction in the power consumption remains difficult with both of the above two driving methods.
Accordingly, a method has been proposed, in which, in the dot inversion driving, a storage capacitor provided for each pixel for storing a data signal voltage is utilized to reduce the voltage amplitude required for driving so that significant reduction in the power consumption can be achieved. In this driving method, after writing the data signal voltage to the pixel electrode and the storage capacitor, the voltage of one of the electrodes of the storage capacitor is changed to shift the pixel voltage to the high potential side or to the low potential side, whereby the pixel voltage thus shifted is made to correspond to the data signal voltage in the conventional dot inversion driving. Due to this voltage shift operation, a desired voltage required for display control can be applied to the pixels. Consequently, in the dot inversion driving which is resistant to flicker noise even at a low frame rate, the amplitude of driving voltage can be reduced, so that significant reduction in power consumption can be achieved.
However, in a transmissive type liquid crystal display apparatus which performs dot inversion driving as described in Japanese Patent Laid-Open Publication No. 2003-150127, in order to apply a voltage having a different polarity to the storage capacitor in each adjacent column, two storage capacitor lines are necessary for one line in the column direction. However, because the storage capacitor line is formed by a metal which does not transmit light, the two storage capacitor lines provided within the pixel region cause reduction in the aperture ratio of the pixel.
In addition, in the transmissive type liquid crystal display apparatus as described in Japanese Patent Laid-Open Publication No. 2003-150127, because the storage capacitor electrode is formed within one pixel region, it is necessary to increase the width of the storage capacitor line to thereby increase the electrode area in order to secure necessary capacitor. However, this also results in reduction in the aperture ratio of the pixel.